update: improved help text

Using the term "the repository's working directory" instead of just "working
directory" to make it nearly impossible to confuse it with $PWD.
Also, this new wording better conveys the fact that update applies
to the working dir as a whole (no matter what the $PWD currently is).

mercurial/commands.py

@@ -2822,8 +2822,8 b' def unbundle(ui, repo, fname1, *fnames, '
 def update(ui, repo, node=None, rev=None, clean=False, date=None):
     """update working directory
 
-    Update the working directory to the specified revision, or the
-    tip of the current branch if none is specified.
+    Update the repository's working directory to the specified revision,
+    or the tip of the current branch if none is specified.
 
     If the requested revision is a descendant of the working
     directory, any outstanding changes in the working directory will